Claws, Jaws and Spikes: The Science of the Dinosaur Arsenal

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Claws, Jaws and Spikes: The Science of the Dinosaur Arsenal

From the playground sandbox to the big screen, we love to imagine dinosaurs tearing into one another. The teeth, horns, claws, and spikes that adorn their skeletons must have had some purpose, after all.

Since the time of their scientific discovery in the early 19th century, dinosaurs have frequently been depicted as ferocious creatures often locked in lethal combat. Images of a Triceratops facing down a Tyrannosaurus bring such long-past confrontations to life, but, thanks to a slew of new studies, paleontologists can do much more than just imagine attack and defense in the prehistoric world.

Dinosaur bones are what remain of once living, breathing animals, and through disparate scientific techniques – from biomechanics to bone histology – paleontologists are providing us with an unprecedented view of the lives and biology of these creatures.

Some of these findings, such as the ability of Tyrannosaurus to toss large hunks of meat into the air before biting back down on them, make them even more frightening, while the notion that the “weapons” of many herbivorous dinosaurs were used more for display than defense is causing scientists to rethink what has been assumed about their evolution.

For over a century, Tyrannosaurus rex has represented the biggest and baddest of the predatory dinosaurs. Its tooth-studded jaws still inspire a mix of fear and fascination in many museum halls, and there can be no doubt that this apex predator of the North American Cretaceous had a formidable bite. But new research shows that the secret of the strength of Tyrannosaurus isn’t found in its jaws, but in its neck.

While the small forearms of Tyrannosaurus and its close kin were muscular and could have acted like meathooks in apprehending prey, these dinosaurs primarily used their head and neck to capture and kill other dinosaurs. Indeed, the neck of Tyrannosaurus would have had to withstand the stresses of grappling with struggling hadrosaurs and horned dinosaurs in addition to the regular strains of carrying around such an enormous noggin.

Using scars left on bone by muscle attachments and the anatomy of living birds and crocodiles as a guide, paleontologists Eric Snively of the University of Alberta and Anthony Russell of the University of Calgary created a digital reconstruction of Tyrannosaurus in 2007 to investigate the range of motion and muscular forces the tyrant’s neck would have allowed.

Their reconstruction of the neck muscles of Tyrannosaurus showed surprisingly that they were strong enough to quickly swing that enormous head to the side while attacking prey. It probably didn’t even need to latch on with its tiny forelimbs before the initial, crushing bite.

Even more impressive, they discoveredTyrannosaurus would have been capable of tossing its prey upwards to give the jaw muscles a moment to relax before snapping shut to reposition the food. According to the scientists’ measurements, Tyrannosaurus could have tossed a 110-pound chunk of meat up to 16 feet in the air. This peculiar mode of consumption, known as inertial feeding, is seen among living birds and crocodiles.

Despite the predatory power wielded by tyrannosaurs, they could be quite delicate with their jaws when they wanted to. Although often cast as indiscriminate bonecrushers, tyrannosaurs could be quite judicious with their bites.

Scientists recently found bite marks on the nearly complete skeleton of a large hadrosaur (right) excavated from the Gobi Desert that were likely punctures and scratches likely made by the eastern cousin of Tyrannosaurus called Tarbosaurus (above). In a bit of fossil forensics, paleontologists David Hone of the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing and Mahito Watabe Hayashibara Museum of Natural Sciences in Okayama, Japan determined that the hadrosaur was dead and mostly buried when the Tarbosaurus happened upon it, with just a few parts of its body sticking up above ground.

Rather than chomp through the protruding limb bones and bolting them down, however, the Tarbosaurus used several different bite angles to strip the remaining muscle off the left arm of the hadrosaur, leaving behind a series of scratches and pits. The results appear June 29 in the journal Acta Palaeontologica Polonica.

*Images: 1) Tarbosaurus/Matt van Rooijen. 2) Close-up of bite marks on the on distal end of hadrosaur bone from the Maastrichtian Bugin Tsav locality in Mongolia. Black arrows indicate deep gouges that penetrate the cortex on the end of the bone. White arrows indicate deep puncture marks on the surface of the bone. /*David W. E. Hone.

Deinonychus————-

Deinonychus was puny compared to Tyrannosaurus, but it was a very different kind of predator. It was a member of a group of dinosaurs called dromaeosaurids which are popularly known as raptors. With long arms tipped with fingers bearing recurved claws, a mouth full of serrated teeth, and a sickle-like claw borne on a hyperextendable second toe, Deinonychus has classically been portrayed as a grappler which used its arms and legs to bring down larger prey while acting in a group.

A recently recovered skeleton of the herbivorous dinosaur Tenontosaurus from Wyoming exhibits different kinds of bite marks and was found surrounded by tooth fragments of Deinonychus.

Given its array of weaponry, it has seemed unlikely that Deinonychus was capable of the heavy bite forces exerted by other predatory dinosaurs with large heads and small forelimbs, but the damage done to the right forelimb of the Tenontosaurus skeleton showed that Deinonychus was indeed capable of a bone-puncturing bite.

In a study published July 4 in the Journal of Vertebrate Paleontology, scientists led by Paul Gignac of the University of Florida in Tallahassee used latex to fill in the punctures to create casts of their shape. They were able to determine the holes were probably made by a large, adult Deinonychus holding the Tenontosaurus forelimb in the front, right part of its jaws.

To investigate what kind of pressure was required to produce this damage, the scientists made a replica Deinonychus tooth out of nickel, which was pressed into a series of cow limb bones. The paleontologists found that it took about 4,100 Newtons of force to drive the artificial Deinonychus tooth into the cow bones, similar to the bite of hyenas and lions. They estimate that the back of the dinosaur’s jaw could have exerted up to twice as much force, similar to those recorded from adult American alligators, and much more powerful than previously thought.

That teeth were important components of the predatory dinosaur arsenal is obvious, but the function of the various ornaments seen on many herbivorous dinosaurs has not been as readily apparent. Take the three facial horns of the famous Triceratops. They certainly look like weapons that could have been useful in deterring a hungry Tyrannosaurus, but scientists have also hypothesized that they could have been used for display or even combat with other Triceratops.

Last year, he led a team that studied the patterns of damage seen in Triceratops skulls. The skulls showed high frequencies of damage on the squamosal bone, which makes up the lateral part of the frill, and the jugal bones, which jut out just beneath the eye. The injuries were probably caused by Triceratops going head-to-head in competition.

The ankylosaurs are often called the “armored dinosaurs” for the thick rows of bony osteoderms arranged over their bodies. These osteoderms took many shapes, from rounded scutes to enormous shoulder spikes and tail clubs. But in research published in June in the journal Acta Palaeontologica Polonica paleontologists led by Shoji Hayaski of Hokkaido University in Sapporo, Japan found that some of the armor of these dinosaurs may not have been as well-suited to defense as previously thought.

One ankylosaur called Edmontonia (right and below) had a set of large spikes poking out over its neck and shoulders, and the density of the bone the scientists found inside one such spike suggests they were used for defense. But when the scientists looked at a similar spike from the dinosaur Gastonia (above), they found the bone was thinner and did not appear to have the kind of reinforcement expected for a weapon. And the armor plating of the ankylosaur Saichania was also relatively weak. While these species’ spikes and armor may have had some defensive benefit, the scientists think they were probably more important for competitive posturing or identifying members of the same species.

If the primary function of horns among the ceratopsian dinosaurs was defense, for example, it would be expected that the arrangement of horns would be similar across multiple species, because there would probably have been optimal horn arrangements for reliable protection, and would have improved over time. Instead the dinosaurs show a riot of different horn arrangements, from that of the famous Triceratops to the recently described and extra-spiky Diabloceratops. Furthermore, differences between male and female dinosaurs have been near-impossible to determine on the basis of gross anatomy alone, so it is unlikely that the evolution of such ornaments was driven mainly by sexual selection.

The scientists suggest something as simple as species recognition played an important role in the evolution of bizarre traits. If this were the case, evolution would favor different forms simply so species could easily recognize each other in landscapes populated by many other dinosaurs. This is not to say that defense, sexual display, or other factors did not influence the evolution of these traits at all, but that we need to look beyond questions of function alone to explain how the bizarre structures of dinosaurs evolved.